1. Technical Field
The present invention is related to a sleeper, the design of which has a double objective: the reduction of the aerodynamic load produced by the passage of the train on the ballast bed, as well as the prevention of the ballast particles from being deposited on the sleeper. In this way, the likelihood of the occurrence of the ballast lifting phenomenon is reduced. The invention is applicable in the railway field; it especially applies in the construction and renovation of railway tracks through which high speed railway vehicles run.
2. Description of the Related Art
Of the elements that make up the track, the sleepers perform different functions in the track in which they are placed, including: the transmission of dynamic stress generated during the transit of railway vehicles, they fix the rails on their place of use, they keep the distance between the rails constant (the width of the track), they preserve the runway and dampen the vibrations of the rail and the acoustic impact on the environment.
The evolution of the materials forming the sleeper has been constant, starting from a sleeper that was constituted by wood, passing through sleepers constituted by two concrete pieces joined together by a metallic element and coming to a sleeper consisting exclusively of concrete, which is the one currently used in the construction of railway tracks.
In the configuration of a track built on ballast, in addition to the electrification and safety systems, the area of the track on which the railway vehicle is supported is made up of rails, sleeper and ballast.
The ballast is the stone material on which the sleepers are placed and that, as well as acting as a support for the sleepers, is used to wrap said sleepers and prevent the lateral movement of the sleepers.
The development of high speed in the circulation of trains has resulted in the occurrence of a series of phenomena that did not occur in the movement of trains at conventional speeds.
One of the new phenomena is ballast lifting. This phenomenon occurs with the movement of the railway vehicle at high speed which, due to the speed of the air that the railway vehicle moves in its movement, transmits a thrust to the stones forming the ballast layer that makes the stones rotate and move, sometimes colliding with the bottom of the train. When the collision takes place, the stone of the ballast is thrown producing collisions with other stones, with the rail, with the sleepers or with other elements of the railway superstructure.
In order to try to prevent the ballast lifting phenomenon, the improvement of the aerodynamic conditions is a key feature, as much from of the train as from the track.
For the improvement of the aerodynamic conditions of the track, the shape of the sleeper is an important factor, which significantly affects the speed of the wind over the ballast bed, thus being a parameter relevant to the ballast lifting phenomenon.
In the realisation of a track, once the platform on which the track will be placed is built, the steps followed are:                a ballast layer (stone material) is placed,        the sleepers are placed on said ballast layer, and        the rails on which the train will move are placed and attached to said sleepers.        
Once the ballast, the sleepers and the rails are placed, the rails are placed in their final position with specialized track mounting equipment (the level is changed and they are moved laterally when necessary), carrying out, at the same time, the wrapping up of the sleepers with ballast, obtained from the one placed first or from a new contribution. The action of wrapping up the sleepers with ballast results in the sleeper being embedded inside the ballast, sticking out only a part of the sleeper (upper faces of the sleeper). The shape of the sleeper, in particular of the part of the sleeper that sticks out from the ballast when the track is ready for the movement of trains, has a significant influence on the ballast lifting phenomenon.
GB 706587 discloses a sleeper consisting of two concrete elements on which the rails are located, connected by a metal piece with different sections; ES 2016883 A6 discloses a sleeper that can be adapted to the two different track widths existing in Spain through a metal piece, by rotating the metal piece, and said piece is attached to the sleeper.